The thermodynamics of amorphous phases in immiscible systems: The example of sputter-deposited Nb–Cu alloys

Abstract

Amorphous metallic alloys, frequently observed to occur in systems with large negative heats of mixing, are much less common in systems which are immiscible in the equilibrium solid state, such as Nb–Cu. However, amorphous Nb–Cu alloys can be produced over a wide composition range by sputtering. Using isothermal and nonisothermal differential scanning calorimetry, both the kinetics and the thermodynamics of these amorphous Nb–Cu alloys were characterized quantitatively. It was found that the formation enthalpies of the amorphous alloys amounted to only 4.5–7.6 kJ/g atom. These data were combined with a modeling of the thermodynamic functions of the system. The unexpected low enthalpies and Gibbs energies of the amorphous phase demonstrate the thermodynamic stabilization of the liquid phase which develops with undercooling. This is connected with a change of sign in the heat of mixing of the liquid phase, which is positive at high temperatures and negative at low temperatures.